In the operation of nuclear reactors, the nuclear energy source is in the form of hollow zircaloy tubes filled with enriched uranium, typically referred to as fuel assemblies. When the energy in the fuel assembly has been depleted to a certain level, the assembly is removed from the nuclear reactor. At this time, fuel assemblies, also known as spent nuclear fuel, emit both considerable heat and extremely dangerous neutron and gamma photons (i.e., neutron and gamma radiation). Thus, great caution must be taken when the fuel assemblies are handled, transported, packaged and stored.
After the depleted fuel assemblies are removed from the reactor, they are placed in a canister. Because water is an excellent radiation absorber, the canisters are typically submerged under water in a pool. The pool water also serves to cool the spent fuel assemblies. When fully loaded with spent nuclear fuel, a canister weighs approximately 45 tons. The canisters must then be removed from the pool because it is ideal to store spent nuclear fuel in a dry state. The canister alone, however, is not sufficient to provide adequate gamma or neutron radiation shielding. Therefore, apparatus that provide additional radiation shielding are required during transport, preparation and subsequent dry storage.
The additional shielding is achieved by placing the canisters within large cylindrical containers called casks. Casks are typically designed to shield the environment from the dangerous radiation in two ways. First, shielding of gamma radiation requires large amounts of mass. Gamma rays are best absorbed by materials with a high atomic number and a high density, such as concrete, lead, and steel. The greater the density and thickness of the blocking material, the better the absorption/shielding of the gamma radiation. Second, shielding of neutron radiation requires a large mass of hydrogen-rich material. One such material is water, which can be further combined with boron for a more efficient absorption of neutron radiation.
There are generally two types of casks, transfer casks and storage casks. Transfer casks are used to transport spent nuclear fuel within the nuclear facility. Storage casks are used for the long term dry state storage. Guided by the shielding principles discussed above, storage casks are designed to be large, heavy structures made of steel, lead, concrete and an environmentally suitable hydrogenous material. However, because storage casks are not typically moved, the primary focus in designing a storage cask is to provide adequate radiation shielding for the long-term storage of spent nuclear fuel.
One type of known storage cask is a ventilated vertical module (“VVM”). A VVM is a massive structure made principally from steel and concrete and is used to store a canister loaded with spent nuclear fuel. VVMs stand above ground and are typically cylindrical in shape and extremely heavy, weighing over 150 tons and often having a height greater than 16 feet. VVMs typically have a flat bottom, a cylindrical body having a cavity to receive a canister of spent nuclear fuel, and a removable top lid.
In using a VVM to store spent nuclear fuel, a container loaded with spent nuclear fuel, such as a multi-purpose canister (“MPC”), is placed in the cavity of the cylindrical body of the VVM. Because the spent nuclear fuel is still producing a considerable amount of heat when it is placed in the VVM for storage, it is necessary that this heat energy have a means to escape from the VVM cavity. This heat energy is removed from the outside surface of the MPC by ventilating the VVM cavity. In ventilating the VVM cavity, cool air enters the VVM chamber through bottom ventilation ducts, flows upward past the loaded MPC, and exits the VVM at an elevated temperature through top ventilation ducts. The bottom and top ventilation ducts of existing VVMs are located circumferentially near the bottom and top of the VVM's cylindrical body respectively.
While it is necessary that the VVM cavity be vented so that heat can escape from the MPC, it is also imperative that the VVM provide adequate radiation shielding and that the spent nuclear fuel not be directly exposed to the external environment. The inlet duct located near the bottom of the VVM is a particularly vulnerable source of radiation exposure to security and surveillance personnel who, in order to monitor the loaded VVMs, must place themselves in close vicinity of the ducts for short durations.
Existing VVMs are made of a dual metal shell structure with shielding concrete inside. The density of concrete can be increased in certain applications to the extent necessary to increase the dose attenuation. Increasing the density of concrete is an effective way to reduce dose. Calculations in specific cases show that increasing the density of concrete from 150 lb/cubic feet to 200 lb/cubic feet reduces the accreted dose from a VVM by a factor as high as 10. However, circumstances arise where it is desired to drive down the local area dose rate from one or more VVMs at an Independent Spent Fuel Storage Installation (ISFSI) to a value which is even smaller than that obtainable by using locally available high density concrete. Such a situation may arise, for example, if local or state authorities impose even more stringent dose rate limits than those specified in 10CFR72, or if there is an inhabited space (say, an office building) close to where the loaded casks are arrayed.